Significant Changes

[#11] Added guidance on solution architecture when developing for Sitecore Experience Commerce and other additional services. Thanks to @robearlam.
[#20] Rewrote and restructured the Introduction section of the documentation to reinforce the intention, scope, core principles, and "why" of Sitecore Helix.
[#20] Added an appendix section reinforcing the relationship of Robert Martin's package principles to specific Sitecore Helix practices. Thanks to @dsolovay.
[#21] Added an appendix section discussing alternate/consolidated Visual Studio solution structures.
[#33] Updated many example references to use Sitecore Helix Examples instead of Habitat.

New/Removed Information

[#8] Updated references to dynamic placeholders to reflect their inclusion in the product.
[#9] Updated guidance on configuration files to reflect added configuration patch capabilities for layers and roles.
[#26] Updated guidance on template GUID constants to use static classes instead of structs.